The principal objective of the proposal is to determine the cellular (neuronal) basis of behavior. We plan to study invertebrates with relatively few neurons. Our approach depends on the development of an apparatus that will simultaneously monitor activity in many (50-100) neurons. This apparatus is based on the fact that optical properties of neurons change when membrane potential changes. If such an apparatus can be developed, it would allow simultaneous monitoring of a large fraction of the total number of neurons in certain invertebrate ganglia. This capability should facilitate the determination of the wiring diagram of the ganglia and, in combination with experiments on semi-intact preparations, should allow a more complete understanding of how nervous systems control behavior and what happens in nervous systems when behavior changes.